Why is glass transparent? An excellent little TED-Ed video, with the science behind it by Mark Miodownik. “If you look through your glasses, binoculars or a window, you see the world on the other side. How is it that something so solid can be so invisible? Mark Miodownik melts the scientific secret behind amorphous solids.” (YouTube)

In short: silicon and oxygen react together to form silicon dioxide, which arranges together in a crystal to form quartz. This is the main component of sand. When quartz is heated strong enough, extra energy makes the crystals vibrate, breaking the bonds holding them together, resulting in a flowing liquid. When this cools, it loses energy, and forms an amorphous solid – a solid material with the chaotic structure of a liquid. The silicon dioxide molecules form in a uniform structure, so light can strike it without scattering. A photon of visible light is also unable to provide enough energy for electrons to jump out of their orbit, allowing it to pass straight through.